Method of reducing materials



Feb. 25, 1958 D. WESTON mom or REDUCING MATERIALS 3 Sheets-Sheet 1 Filed May 25, 1954 INVENTDR DA VID WESTON COARSE FINES AND suMss ATTORNE} s CHARGE VOLUME Feb. 25; 1958 D, W STON 2,824,700

METHOD OF REDUCING MATERIALS Filed May 25, 1954 s sheets-sheet s 40- i 4-5 CONTROL.

42 43 44 SIGNAL 5 c'cwmmmk AMPL/F/ER 'Zg'ggg E a REFERENCE SIGNAL f INVENTOR DAVID WESTON aniMna k/fi ATTORNE r5 Unit! Stews Patent 2,824,700 METHOD on REDUCING MATERIALS David Weston, Toronto, Ontario, Canada Application May 25, 1954, Serial No. 432,200 3 Claims. (Cl. 241-19) The present invention relates to a method of operating a combined dry crushing and grinding mill. The invention relates more particularly to a method of operation of such mills for purposes of producing a maximum proportion of product within a selected size range. In my prior Patent No. 2,555,171 and in my prior application Serial No. 315,470 filed October 18, 1952 (now U. S. Patent No. 2,704,636), I have disclosed a combined dry crushing and grinding mill which is characterized by the removal of the product in a current of air and by a miil drum having a diameter length ratio of at least 2:1 and which is provided with highly upstanding transverse crusher bars uniformly spaced about the interior periphery thereof. It is to the operation of mills of this type that the method of the present invention has its application.

One of the chief features of mills of this type is that the comminution within them makes use of not only several fundamentally new principles but also to a greater or lesser degree a number of previously known principles of material reduction; that is to say crushing by impact and free fall, and grinding by abrasion.

In my earlier use of such mills, I had appreciated the presence of the majority of these factors and had generally sought to operate the mill in such manner as to take advantage of the combination of most of them to obtain the maximum eificiency of the mill for various specific applications. As indicated in my said application as well as in my copending application Serial No. 234,782 filed July 2, 1951 (now U. 8. Patent No. 2,680,568), I determined that the maximum capacity of such mills could be attained by rotating the mill within the range of from 82-90% of critical speed and maintaining a charge volume in the mill of from 24-32% of the total mill volume, the ideal load being generally of the order of approximately 27% of the mill volume. Such conditions were adapted to bring about a maximum application of the crushing factor and lesser application of the grinding and other factors which are present and the results produced indicated a high degree of effectiveness in the production of mill products in the ultimate natural grain size distribution. It is frequently desired, however, to produce products as aggregates of primary crystals or products in which the primary crystals have further been broken down to a fine state of subdivision. In addition, a number of materials which are commonly subject to commercial reduction do not have any welldefined primary crystalline structure; yet it may be desired to produce a mill product from these which is predominantly within a particular particle size range.

Subsequent studies of the manner in which the grinding factors in such mills are affected by various conditions have revealed that in the operation of a mill of the abovedescribed type, it is possible to control the relative application of the crushing and grinding factors to produce products which are predominantly of a predetermined andvdesired state of subdivision.

2,824,700 Patented Feb. 25, 1958 I have found that for given mill speeds the application of the grinding factor is primarily determined in direct proportion to the charge volume in the mill where- 1 as the crushing factor is dependent upon rather critical" limits of charge volumes and will be reduced materially as the charge volume is varied to a substantial extent in either direction. Thus, when the grinding factor is at a maximum, the crushing factor is at a relatively low value and when the crushing factor is at a maximum, the grinding factor is at a value considerably below its own maximum. By varying the total load in the mill, it is possible to operate with a desired relative influence of the crushing and grinding factors, and thus produce products to a wide range of specifications at the will of an operator.

In a combined dry crushing and grinding unit of the type described above, all of the particles within the charge Within certain zones will tend during operation of the mill to rotate on an axis of their own which is parallel to the axis of rotation of the mill under the influence of what is known as the induced couple which arises as a result of the rotation of any body about an exterior axis. The grinding action of the mill occurs primarily as a result of these rotating particles abrading on one another. I have found that the charge volume itself is the main factor in controlling the forces within the mill to produce a desired fineness of grind and that if such charge volume is increased, the comminution forces are increased and the amount of fine grinding work which is done increases to a corresponding extent. At the same time, if the volume of the charge is increased to an extent where the toe of the charge extends substantially beyond a plane extending vertically through the axis of the mill, the basic crushing action which takes place is greatly reduced as a result of the dampening of crushing blows administered by the crushing bars in the manner described in my Patent No. 2,680,568, and finally also the grinding efiiciency drops oil? as the particles can no longer turn on their axes.

The above observations are in direct contrast to the accepted theory of conventional ball mills and the like wherein the degree of reduction effected on each particle is basically determined by the angle of nip between adjacent grinding media regardless of the charge volume (within the normal operating volumes of 35% to of the mill volume) and wherein the charge volume basically determines the rate of production of the mill.

The invention and its operation will be described in greater detail in conjunction with the accompanying trawings wherein:

Figure l is a diagrammatic cross section taken through the central plane of a combined dry crushing and grinding mill of the type to which the method of the invention a has applications;

Figure 2 is a general view partly diagrammatic of a typical grinding circuit embodying the combined dry crushing and grinding mill of the type referred to;

Figure 3 is a graph illustrating the variation in particle size produced for difierent charge volumes in a typical test run using serpentine peridotite as the feed material;

Figure 3a is a graph illustrating the general shape of a grinding curve for a combined dry crushing and grind ing unit of the type referred to herein;

Figure 4 is a block diagram illustrating a suitable control system for use in connection with the invention.

Referring more particularly to the drawings, from Figures 1 and 2, it will be observed that the drum 10 of the combined dry crushing and grinding unit is characterized by a large diameter length ratio, the diameter in all cases being at least twice the length thereof. At the same time, the interior of the periphery of the drum carries a plurality of highly unstanding crusher bars 11 uniformly spaced apart such a sufiicient distance that material undergoing reduction in the mill will not tend to wedge between the crusher bars and be carried around on the periphery of the drum. In addition, various liner elements may be carried by the end walls of the drum such as elements 12 (see Figure 2) to accomplish certain particular functions. These liners and their operation are described more particularly in my prior Patent No. 2,555,171 and my Patent No. 2,704,636.

The drum is provided with the central openings 13 and 14. Material is charged to the mill through the opening 13 and leaves the mill through the opening 14, through which opening also a certain amount of withdrawn oversize material may be returned to the mill. Throughout operation of the mill, a current of air motivated by the fan 15 is drawn through opening 13 across the mill and out through opening 14. The velocity of this current of air is such that it will entrain and carry out of the mill the desired particle size of mill product. After passing out of the mill, the air stream and entrained product pass around the elbow 16 where oversize material is dropped from the air stream and flows back down the surface 17, through the opening 14 and back into the mill for further reduction, while the main mill product remains in the air stream and is seperated therefrom in the cyclone 18 from which it is discharged through a rotary air lock 19 as final mill product. The air stream, which will still contain a certain amount of very finely divided material passes along through the fan 15 and is recirculated through duct 20 back into the inlet side of the mill. In order to control the dust and moisture content of the recirculated air, a certain amount of it is withdrawn from the recirculating duct 20 through the dust collector 22 by the fan 21 and discharged to atmosphere while sutficient fresh air is added at point 23 to maintain equilibrium conditions in the air circuit and control dust and moisture conditions within drum 10. This air circulating system is described in my prior patent application Serial No. 253,399, filed October 26, 1951 (now Patent No. 2,674,413). It will be understood, however, that the present invention does not depend upon the particular type of air circulating system used, that de scribed being used for purposes of illustration only.

During operation of the mill the particles of material undergoing reduction generally tend to follow a natural path within the drum depending upon their size. This tendency is illustrated in Figure l, which indicates the theoretical paths which would be followed by balls of various diameters, assuming that each ball were free to travel in its preferred path. A similar situation prevails with particles of charge and, generally speaking, the largest size particles will tend to concentrate near the toe of the charge and will not be carried as high up in the charge on the upwardly moving side of the drum as will the smaller particles. As the particles become smaller they are carried higher and higher until, when they have arrived at a size of approximately 1", they are carried out through the top of the charge and tend to follow a free fall path in what is labelled the cataracting zone in Figure 1. Naturally, in actual operation, the mechanical contact of particles of various sizes prevents complete segregation of the type which might be indicated by the drawing in Figure l, but nevertheless the diagram indicates the tendency which is present and which is believed to explain in some measure thesurprising results achieved in accordance with the present invention.

Once again, referring to Figure 1, it will be observed that there is a cascade zone which forms on top of the charge through which particles of approximately 1 and larger in size will tumble back towards the bottom of the mill on the top of the charge, arriving there in the region of the toe 30. At the same time there is a false toe 31 formed immediately in front of the toe 30 by feed material which is constantly entering the mill and 7 dropping to the bottom thereof. As explained in my Patent No. 2,680,568, the principal crushing effect which takes place in the mill occurs in the region of the toe by virtue of the crusher bars 11 driving into the toe which contains the largest particles of charge and therefore the greatest inertia. The effectiveness of the crushing action depends firstly upon the fact that these larger particles and/ or balls are constantly arriving in the region of the toe while moving vertically downwardly towards the shell of the drum and are continuously being struck by the faces of the crusher bars 11. It will be appreciated that this maximum crushing eifect will only prevail while the toe area of the charge and the false toe 31 lie substantially astride the vertical line 32 drawn through the centre of the drum. It will also be apparent that the position of the toe of the charge will be dependent upon the total charge volume within the mill. The ideal charge volume for effecting maximum crushing effect is generally illustrated in the diagram and corresponds generally to approximately 27% of the total volume of the mill drum itself. If the charge is diminished substantially from this value the toe 30 will move to the left and the false toe because it is formed of material dropping vertically in through the inlet opening 13 remains in its central location. The whole charge becomes less dense and the crushing action described is accordingly less effective. On the other hand, if the charge volume is increased substantially beyond the ideal as represented for maximum capacity, the toe will move to the right, merging with the false toe 31; the material in the cascade zone will increase in quan tity resulting in a considerable amount of material flowing over the toe and mingling with the material from the cataracting zone to, in effect, form a cushion of relatively small size material and prevent the crusher bars 11 from striking the large size material in the toe and false toe with their bare faces. The result, of course, is a considerable reduction in the effectiveness of the crushing blows administered by the crusher bars. At the same time, however, the size of the abrasion zone has been increased and the length of time that any individual particle will remain in the charge has been increased since it takes longer for a particle to pass through a charge of large volume than one of small volume. In addition the charge has become more compact both due to the greater weight of material contained within it and due to a lessening in the supporting effect of the liner elements 12 which produce what has been termed a keying action, which effect is described in greater detail in my Patent No. 2,704,636. The result is that whereas the crushing action has been substantially reduced the amount of true grinding work is also reduced with slippage rather than turning of the particles on their axes resulting in sliming or other fine comminution which in many cases is an undesirable product, the net result being a lower capacity in tons per hour, a product of a slimey nature and an increase in power consumption compared to operation of the mill at the point of optimum capacity where the crushing factor is at a maximum.

The variation in controlled product particle size produced by variations in charge volume is well illustrated in Figure 3, which is a graph of a test in which a combined crushing and grinding mill of the type described having nominal dimensions of 17 in diameter and 5 in length was used in the comminution of serpentine peridotite, from a mine in the eastern part of Quebec Province, Canada.

In the course of this test the mill was operated with an electronic control enabling the maintenance of any desired operating conditions and experimental data for a number of operating points were tabulated and used as the basis for grabs A, B and C which appear in Figure 3. A constant air velocity through the mill was held for the complete series of tests.

Graph A represents the capacity of the mill in tons per hour plotted against H. P. drawn by the mill motor.

It will be seen that graph A (which we shall refer to herein as the capacity curve) reaches a maximum at approximately 330 H. P. and then declines in almost linear relationship to increase in H. P. until a rapid falling off occurs in the region of 520 to 530 H. P. The maximum point M of the capacity curve represents the point of maximum capacity of the mill and the point of maximum crushing effect wherein conditions within the mill can be represented in general terms by the Figure 1. The rapid falling off which occurs beginning approximately at point P on the capacity curve is a result of the charge in the mill becoming so large that the crushing factor has been seriously affected and substantial slippage of the charge within itself has occurred.

For purposes of comparison curve A is shown in dotted lines and represents a typical capacity curve for a conventional ball mill in which the capacity at the same grind screen analysis gradually increases to a maximum which is quite close to a rapid fall 01f in capacity.

Graphs B and C are graphs of percentage particle sizes obtained, plotted on the same power base as curve A. Curve B represents percentage minus 20 mesh and curve C represents percentage plus 4 mesh. It will be observed that these two curves cross each other at 340 H. P. and that thereafter the percentage of minus 20 mesh increases as the power consumption increases while the percentage plus 4 mesh decreases until at approximately 480 H. P. the product is substantially 100% minus 4 mesh.

In conventional dry grinding mills, curves corresponding to curves B and C and related to curve A would be substantially parallel and horizontal since the distribution of particle sizes in the product of such mills basically depends upon the type of reduction media used and not upon the charge volume within the mill as has been shown to be the case with combined dry crushing and grinding units of the type, with which this invention is concerned. Appreciation of the factors involved in controlling the grind within combined dry crushing and grinding units of the type described has only been possible subsequent to the development of effective control means for maintaining substantially constant charge volumes within the mill. In mills of this nature, normally the initial breakdown action is very rapid and any decrease or increase in the rate of feed to the mill is rapidly reflected by an increase or decrease in the charge volume in the mill. Manual control methods result in a comparatively large variation in charge volume rendering an appreciation of the variation in product particle size with variation in charge volume most difficult. Accordingly, although I had suspected that some variation in the product of combined dry crushing and grinding units of the type described was related to the volume of the charge, it was not until I developed effective controls of the type described in my copending applications Serial Numbers 259,060, filed November 30, 1951, now abandoned, 282,505, filed April 15, 1952, now Patent No. 2,766,939, October 16, 1956, and 426,721, filed April 30, 1954, now Patent No. 2,766,941, October 16, 1956, that I was able to determine that there was a direct functional relationship between particle size of the product produced by such mills and volume of charge maintained during operation thereof. With the assistance of these controls, which make it possible to maintain a given charge volume even though operating with a greater charge volume than that which produces maximum capacity (charge volume being also, it will be appreciated, a function of mill motor H. P.), I have found that it is possible to operate a mill of this type in a manner which produces products to a relatively wide range of product specifications simply by maintaining during operation a charge volume within the mill which bears a particular predetermined volumetric relationship to the total volume of the mill drum.

As will be appreciated, it is generally not practical in operating according to the method of the invention to measure directly the actual charge volume within the mill, and it is convenient, therefore, to establish desired conditions by reference to the grinding curve, each point whereof represents operation of the mill with a particular charge volume. A typical grinding curve for a combined dry crushing and grinding mill of the type above referred to is shown in Figure 3A and as will be observed is derived by plotting capacity in tons per hour against charge volume. Referring to Figure 3A, it will be observed that the grinding curve reaches a maximum at point X, which point represents the charge volume at which the mill has its maximum capacity. Points on the grinding curve to the left of point X generally speaking produce coarser particle sizes while points to the right of point X represent conditions wherein considerably more fine grinding work is done by the mill. It will be ob-- served that if it is known on which side of point X the desired operating conditions fall, horse power required to operate at the desired point can be determined, and

it is possible to relate the particle size distribution de-- sired directly to the horse power drawn by the mill motor.

It will be appreciated from what has just been said that the desired operating point to produce a particular product must be determined experimentally for each particular type of ore which it is desired to comminute. This will involve in the first instance the determination of on which side of point X on the grinding curve the desired operational point occurs followed by a determination of the location on the grinding curve of the point which will produce the particular product required. It will be appreciated, however, that bearing in mind the nature of variation of product particle size with charge volume, it is a simple matter in each case to determine the desired operating point which must be maintained to produce the product desired. It is of particular importance to note the control of slimes which can be accomplished according to the present invention. By operating to the left of point X, a product containing very little or no slimes can be produced while operation to the right of the point X on the grinding curve will produce slimes to a controlled extent so that where it is desired to produce a high proportion of slimes this can be done simply by selecting an appropriate point of operation to the right of point X on the grinding curve.

As previously mentioned, suitable control systems for maintaining the volume of the charge in a mill at a desired value are described in my aforementioned copending applications Serial Numbers 259,060 filed November 30, 1951, now abandoned, 282,505 filed April 16, 1952, now Patent No. 2,766,939, October 16, 1956 and 426,721 filed April 30, 1954, now Patent No. 2,766,941, October 16, 1956. In particular, I prefer to use a control system of the type described in the last mentioned of said applications. Such control systems control the feed to the mill-on the basis of a difference signal derived by comparing a control signal which is functional to or which varies with an operating condition of the mill with a reference signal which corresponds to a desired state of said operating condition.

A simplified diagrammatic illustration of such a control is shown in Figure 4. The control signal is produced in means which are represented by the box 40, while the reference signal is produced independently by means represented by the box 41. The two signals are compared in comparator 42 and the difference signal having been amplified in an amplifier 43 is used to control the proportioning feeder 44 which feeds material to the mill 45 at a rate which is increased or decreased in accordance with the sense and magnitude of the difference signal produced in the comparator 42.

' For purposes of the present invention the control signal may be derived either from the sound emanating from the mill by means of a dynamic microphone pickup device or it may be derived from the power drawn by the mill motor by means for instance of an elec ronic watt meter. In-

stead of using sound as the source of the control signal, vibration may be used and in either event it may be desired to use only a selected band of frequencies in order to eliminate vibration and sound not directly related to the operation being carried on within the drum of the mill. Generally speaking, I prefer to derive a control signal in such manner that it is functional to the sound emanating from the mill which has a frequency of above 2000 cycles per second, which type of sound I have found to be highly satisfactory as representing actual conditions within the drum.

If I use power as my control, I may use a sound signal as a monitor to keep the power on the correct side of the grinding curve. This is only desirable where we desire a slimed product or are working near the peak of the curve.

The selection between power and sound or vibration as a source of the control signal depends primarily on what portion of the capacity curve the operational point which it is desired to maintain occurs. As explained in my said Patent No. 2,766,941 power drawn by the mill motor is a more satisfactory control signal over low charge volumes, whereas sound or vibration is generally more sat isfactory when operating at higher charge volumes. Insofar as the present invention is concerned it will be apparent from a consideration of the graphs in Figure 3 that for fine grinding work operation will usually be maintained wi h a charge volume larger than that which produces maximum capacity, whereas for coarse grinding it may be desired to operate to the left of the point M on graph A. In many cases and particularly if the desired operational point occurs in the region of maximum work input it may be desired to utilize a composite control signal or to use a control signal based on one of the above sources monitored by a second signal derived from the other source. The use of composite control signals is described in my aforementioned copending application Serial Number 259,060 and Patent No. 2,766,939 while the use of a monitored signal is described in my Patent No. 2,766,94l. Whatever control signal is used, the signal is compared to a reference signal of a value representing the desired charge volume at which it is intended to operate and the rate of feed of material to the mill is varied in accordance with the sense and magnitude of the difference signal thus produced. In this way the charge volume within the mill can be maintained constant to within relatively narrow limits, making possible a surprising degree of control over the particle size of the product which is produced.

It will be remembered that the product of dry grinding and crushing mills of the type described is withdrawn in a current of air and it will therefore be apparent that it is important according to the present invention that provision should be made (within limits determined by operating eh'iciency of the mill and the air system) for removing the product which is formed within the size ranges which are desired. Accordingly, in operating according to the method of the present invention, it is necessary to ensure that a current of air is maintained across the drum of the mill which is of a sufficient velocity and in sufiicient volume to entrain and carry away from the mill the product desired. In general it is preferable to operate at a somewhat higher velocity and entrain a certain amount of oversized material which is separated from the air stream at the elbow 16 (see Figure 2) and which then returns to the mill for further reduction.

What I claim as my invention is:

1. In a method of producing a comminuted product of controlled particle size from a combined dry crushing and grinding mill ot' the type wherein air is used as the product extracting medium and comprising a drum mounted for rotation on a horizontal axis, said drum having a diameterlength ratio of at least 2:1 and being provided with highly upstanding transverse crusher bars spaced apart about the interior periphery thereof, said mill being characterized by maximum tonnage throughput at a particular charge volume ranging from about 24% to 32% of the total mill volume, the steps comprising; providing an operating charge volume of ore in said mill determined from established grinding characteristics of said ore in said mill based on the correlation of ore charge volume to average particle size in the resulting mill product wherein controlled coarse particle size is obtainable by maintaining the ore charge volume at a value below said particular charge volume corresponding to said maximum throughput and controlled particle size obtainable by maintaining the charge volume at a higher value than said particular charge volume, supplying feed material to the mill at a rate to maintain said charge volume constant at said operating value thereby to obtain said desired controlled particle size characteristic of said operating charge volume, and maintaining sufficient flow of air through said mill of sufiicient velocity to entrain and carry away from said drum the desired mill product.

2. In a method of producing a comminuted product of controlled particle size from a combined dry crushing and grinding mill of the type wherein air is used as the product extracting medium and comprising a drum mounted for rotation on a horizontal axis, said drum having a diameterlength ratio of at least 2:1 and being provided with highly upstanding transverse crusher bars spaced apart about the interior periphery thereof, said mill being characterized by maximum tonnage throughput at a particular charge volume ranging from about 24% to 32% of the total mill volume, the steps comprising; providing an operating low charge volume of ore in said mill below said particular charge volume, said low charge volume being determined from established grinding characteristics of said ore in said mill based on the correlation of ore charge volume to average particle size in the resulting mill product wherein controlled coarse particle size is obtained by maintaining the operating charge volume at said low value, supplying feed material to the mill at a rate to maintain said operating charge volume relatively constant at said low charge thereby to obtain said desired controlled particle size characteristic of said operating charge volume, and maintaining sufiicient flow of air through said mill of sufficient velocity to entrain and carry away from said drum the desired mill product.

3. In a method of producing a comminuted product of controlled particle size from a combined dry crushing and grinding mill of the type wherein air is used as the product extracting medium and comprising a drum mounted for rotation on a horizontal axis, said drum having a diameter-length ratio of at last 2:1 and being provided with highly upstanding transverse crusher bars spaced apart about the interior periphery thereof, said mill being characterized by maximum tonnage throughput at a particular charge volume ranging from about 24% to 32% of the total mill volume, the steps comprising; providing an operating high charge volume of ore in said mill above said particular charge volume, said high volume being determined from established grinding characteristics of said ore in said mill based on the correlation of ore charge volume to average particle size in the resulting mill product wherein controlled fine particle size is obtained by maintaining the operating charge volume at said high value, supplying feed material to the mill at a rate to maintain said operating charge volume relatively constant at said high value thereby to obtain said desired controlled particle size characteristic of said operating charge volume, and maintaining sufficient flow of air through said mill of sufficient velocity to entrain and carry away from said rum the desired mill product.

References Cited in the file of this patent UNITED STATES PATENTS 2,001,543 Payne May 14, 1935 (Other references on following page) Roder Nov. 15, 1938 Hardinge Aug. 7, 1945 Beach Apr. 23, 1946 Weston May 29, 1951 5 10 Weston Aug. 28, 1951 Weston June 8, 1954 Weston Oct. 16, 1956 Weston Oct. 16, 1956 Weston Oct. 16, 1956 

